1. Field of the Invention
The present invention relates to an electric power steering apparatus, and more particularly to an improvement in a rack and pinion mechanism used in such an electric power steering apparatus.
2. Description of the Related Art
Electric power steering systems are commonly used to make steering easier by reducing a force needed to turn a steering wheel (referred to as the steering force below). Electric power steering systems use an electric motor to produce assist torque according to the steering torque, and transfer this assist torque to the rack and pinion mechanism of the steering system, as taught in, for example, Japanese Patent Laid-Open Publication (kokai) No. HEI-9-193815.
More specifically, the electric power steering system produces assist torque according to the steering torque by means of an electric motor, transfers this assist torque through small and large bevel gears to a rack and pinion mechanism comprising a pinion and rack shaft, and steers the steering wheels by means of this rack and pinion mechanism. The rack shaft is a round rod having a rack formed thereon at the place opposite the pinion. The pinion and rack both have involute teeth.
An automotive steering system also usually has a stopper mechanism for limiting the maximum turning angle of the steering wheels. More specifically, this stopper mechanism has a rack end stopper attached at each longitudinal end of the housing in which the rack shaft is slidably disposed, and a ball joint, for example, is attached to each end of the rack shaft. When the rack shaft slides a specific distance, the ball joint contacts the rack end stopper. The maximum turning angle of the steering wheels is thus limited by limiting the movement of the rack shaft.
The rack and pinion of the rack and pinion mechanism used in the electric power steering apparatus taught in Kokai HEI-9-193815 uses spur or helical gears. The tooth profile of these spur or helical gears is also involute. Helical gears are widely used in high load, high speed gear applications because they mesh more smoothly than spur gears and produce less vibration and noise.
Small, high load helical gears are also used in the rack and pinion mechanism of the above-noted electric power steering apparatus. Helical gears produce a constant thrust corresponding to torque because the tooth profile has a specific helix angle. The thrust produced during normal steering conditions is determined by the total torque acting on the pinion, that is, the steering torque applied by the driver plus the assist torque produced by the motor.
Once the rack shaft slides the above-noted specific distance, further movement is restricted by the stopper mechanism. When the rack shaft is thus stopped, the total torque acting on the pinion is greater than during normal steering, and high thrust corresponding to this total combined torque is at work. Note that maximum combined torque and maximum thrust are produced at this time.
The power transfer section, bearings, housing, and other components of the electric power steering apparatus must also be strong enough to withstand this maximum thrust. Increasing the strength of these components requires relatively high quality materials and relatively large components. The electric power steering apparatus itself thus becomes larger and more expensive, leaving room for improvement.
In the above-noted electric power steering apparatus the assist torque (assist force) produced by the motor is increased by a reduction mechanism comprising small and large bevel gears, and the increased assist force is converted to thrust assistance by the rack and pinion mechanism. The assist force of the motor is converted to rack output at the combined efficiency xcex7T=xcex7Gxc3x97xcex7R where xcex7G is the transfer efficiency of the reduction mechanism and xcex7R is the transfer efficiency of the rack and pinion mechanism. The product of motor assist force and (1xe2x88x92xcex7T) is output loss, which is converted to parts wear and heat, and contributes to a drop in system durability and output due to heat.
The effect of output loss is particularly great, and it is therefore desirable to improve transfer efficiency xcex7G and transfer efficiency xcex7R, in electric power steering systems that convert motor assist force from a high output motor to rack thrust by way of a reduction mechanism and rack and pinion mechanism.
It is therefore a first object of the present invention is to provide a compact, low cost electric power steering apparatus having a rack and pinion mechanism with durability sufficient to withstand the torque load of motor inertia.
A second object of the present invention is to provide particularly technology for improving the transfer efficiency of the rack and pinion mechanism.
A third object of the present invention is to provide an electric power steering apparatus having a rack and pinion mechanism with sufficient strength relative to motor inertia by maintaining good mesh between the pinion and rack.
To achieve the above objects, an electric power steering apparatus according to the present invention has a motor for producing an assist torque in correspondence with a steering torque, a rack and pinion mechanism for a steering system, and a geared reduction mechanism for transferring the assist torque to the rack and pinion mechanism. The pinion and rack of the rack and pinion mechanism are both helical gears. The helix angle of the pinion is less than the helical gear friction angle. One of the helical gears has a tooth profile wherein at least the addendum is a circular arc substantially centered on the reference pitch line. The other of the helical gears has a tooth profile wherein at least the dedendum is a circular arc practically centered on the reference pitch line.
By using helical gears, the rack and pinion mechanism can transfer higher torque than a conventional spur gear.
When the steered wheels turn right or left to the maximum steering angle and the rack shaft meets the rack end stopper, that is, when the rack shaft moves to the end of its range of movement, the rack drops immediately. Because the torque at this time is impact torque and not static torque, torque is significantly higher than during normal driving conditions. However, because the helix angle of the helical gear pinion is less than the helical gear friction angle, thrust does not act on the pinion. Thrust acting on the pinion is only an extremely weak force occurring during normal conditions when the rack is not stopped at the right or left end of its range. Thrust acting on the input shaft is therefore low, and thrust acting on the bearings supporting the input shaft and the geared reduction mechanism linked to the input shaft is low. It is therefore not necessary to increase the strength of the input shaft, bearings, and geared reduction mechanism even though helical gears are used. These components can therefore be downsized and less expensive.
The tooth profile of the pinion and rack of the rack and pinion mechanism of the present invention is a curved arc. Because a conventional involute tooth profile is convex, meshing in a gear pair is contact between two convex surfaces. With the curved arc tooth profile of the present invention, however, meshing in a gear pair occurs as contact between a convex surface and a concave surface. The contact area is thus increased, and contact pressure is reduced to approximately ⅙ that of an involute tooth profile.
By thus using a curved arc tooth profile in the rack and pinion of the rack and pinion mechanism, surface fatigue strength, bending strength, and bending fatigue strength are greater than with an involute tooth profile. This means that the rack and pinion mechanism of our invention can transfer the combined torque achieved by adding the assist torque from the motor to the steering torque, even when this combined torque is greater than during normal conditions.
The present invention can thus provide a compact, low cost electric power steering apparatus having a rack and pinion mechanism with durability sufficient to withstand torque loads resulting from motor inertia.
It is further preferable to insert a torque limiter between the motor and the geared reduction mechanism to limit the transfer of assist torque exceeding a specific limit from the motor to the reduction mechanism. When the rack shaft hits the rack end stopper, excessive torque will not be produced as a reaction to the motor, and excessive torque will not be transferred to the load side.
It is yet further preferable to provide a steering torque sensor for detecting steering torque. Yet further preferably the steering torque sensor is a magnetostrictive sensor for detecting magnetostriction of the pinion shaft of the rack and pinion mechanism. By using such a steering torque sensor, it is not necessary to divide the input shaft into two parts lengthwise and connect these two parts using a torsion bar as it is when steering torque is detected using the method of a conventional electric power steering apparatus. It is therefore also possible to lengthen the input shaft. Machining precision is increased by lengthening the pinion shaft, and the pinion and rack thus mesh more precisely. There is a particularly strong correlation between meshing precision and power transfer efficiency in a rack and pinion mechanism having a curved arc tooth profile, and improving meshing precision is therefore important.
The geared reduction mechanism of the present invention is preferably a combination of driver and driven gears in which the tooth surfaces of the driver gear and/or the tooth surfaces of the driven gear are coated with a low friction material coatings and the driver gear and driven gear mesh with no backlash. Coating with a low friction coefficient material can be achieved by imparting a coating made from a low friction coefficient material, or by impregnating the tooth surfaces with a low friction coefficient material.
By thus meshing driver gear and driven gear with no backlash, there is no play between the driver and driven gears, and impact torque due to motor inertia does not pass from the driver gear tooth surface to the driven gear tooth surface.
Moreover, the tooth surfaces of one or both of the driver gear and driven gear are coated with a low friction coefficient material coating. By lowering the coefficient of friction between the tooth surfaces of the driver and driven gears by means of this coating, power transfer efficiency can be increased even though there is no play between the driver and driven gears.
The pinion and/or rack of the rack and pinion mechanism in the present invention is yet further preferably a forging or other plastically processed part. There are, therefore no process marks left on the tooth surface as there are when the tooth surfaces are conventionally machined, and the surface roughness of the gear teeth is smooth. Friction force from sliding fear tooth surfaces is thus reduced, and the power transfer efficiency of the rack and pinion mechanism is increased.
Furthermore, because the pinion and rack are plastically processed parts, there is no residual stress produced in the tooth surfaces as there is with machining processes, and there is thus less deformation during hardening. A good tooth surface with low strain can therefore be achieved without correcting the tooth profile after hardening. In other words, because these parts are plastically processed, the surface roughness condition of the teeth is good with little strain from hardening or tool marks left. In addition, strength is increased because a fiber structure flowing continuously along the tooth profile is achieved through plastic processing, and bending strength and wear resistance are greater compared with machined gears in which the fiber structure is interrupted (not shown).
By processing the teeth of the rack and pinion to a curved arc tooth profile, and achieving this curved arc tooth profile in the rack and pinion by means of forging or other plastic processing technique, contact pressure is reduced, a good surface roughness condition is achieved, and interruption of the oil membrane formed by the lubricating fluid can be prevented. An electric power steering apparatus with little motor output loss can thus be provided because contact resistance between tooth surfaces can be significantly reduced and the power transfer efficiency of the rack and pinion mechanism improved.
Furthermore, by using forgings or otherwise plastically processed components for the curved arc tooth profile pinion and rack, it is possible to provide an electric power steering apparatus featuring improved mechanical properties in the materials, less tooth base stress, reduced wear, and outstanding strength and durability.
Yet further preferably, the rack shaft to which the rack is formed is comprised so that the back on the side opposite that to which the rack is formed is pushed toward the pinion by an adjustment bolt by way of intervening rack guide member and compression spring, particularly so that the adjustment bolt pushes directly against the back of the rack guide member when the pinion and rack mesh.
Good meshing between the pinion and rack can be maintained as a result of the rack guide member constantly pushing the rack shaft to the pinion, and the power transfer efficiency of the rack and pinion mechanism can thus be stabilized. Assist torque from the motor can be particularly transferred efficiently from the pinion to the rack shaft even during high load conditions such as turning the wheels when the vehicle is stopped. Therefore. compared with using a conventional involute tooth profile, less assist torque is needed, and a low power consumption electric power steering apparatus can be provided.
Moreover, tooth surface wear is reduced because the curved arc tooth profile is formed by forging or other plastic processing method. It is therefore possible to provide an electric power steering apparatus having a rack and pinion mechanism with little play even without applying pressure using an adjustment spring.
Furthermore, because the tooth profile of the rack and pinion is a curved arc as described above, the contact area of meshed teeth is greater than that with an involute tooth profile. Because the contact pressure drops, tooth surface sliding is also smoother. A good steering feel can also be maintained in the steering wheel even though an adjustment bolt directly supports the rack shaft so that the rack shaft will not move back in reaction to the strong force produced perpendicular to the longitudinal axis when high torque due to motor inertia acts on the rack and pinion mechanism.
The tooth width of the rack formed on the rack shaft in the present invention is greater than the diameter of the rack shaft in the part where the rack is not formed.
The rack shaft can be made from round rod or pipe stock.
The rack shaft on which the rack is formed is housed unrockably and slidably in the longitudinal direction in a housing. A rocking force is produced on the rack shaft when the pinion and rack are helical gears, but this rocking action of the rack shaft is restricted in the present invention. Good meshing between the pinion and rack can thus be maintained.
More specifically, the back of the rack shaft opposite the surface on which the rack is formed is convex, and a rack guide is disposed having a concave end for contacting convex back at contact points, and pushing the convex back of the rack shaft toward the rack. These contact points are set in relation to the rack shaft supported by the housing so the concave end limits rocking of the convex part of the rack shaft when a rocking force acts on the rack shaft. The rack shaft is thereby housed so that it cannot rock in the housing.
The rack guide preferably pushes the guide member having the concave end to the rack shaft side by means of adjustment bolt and intervening compression spring. The adjustment bolt pushes directly on the back of the surface to which the concave end is formed to the guide member when the pinion and rack mesh.
When torque is transferred from the pinion to the rack during steering, forces act on the rack shaft in the direction of the longitudinal axis and in the direction perpendicular thereto. Because the adjustment bolt pushes directly against the back of the guide member, the rack cannot move back as a result of force in the longitudinal axis direction. Good meshing between the pinion and rack can thus be always maintained. Moreover, the contact area is great and contact pressure between meshing surfaces is reduced as a result of the curved arc tooth profile, and sliding between the tooth surfaces is therefore smoother.
Yet further preferably, a supported part whereby the rack shaft is supported on a housing by way of intervening bearings, and a rack formation part to which the rack is formed, are disposed to the rack shaft. The section perpendicular to the axis of the rack formation part is a circular section equal in diameter to the supported part, and the distance from the center of this circular section to the reference patch line is set to a specific dimension. The actual tooth width of the rack is greater than the rack tooth width determined by this specific dimension.
By thus making the tooth width of the rack actually greater than the tooth width of a conventional rack, the mechanical strength (bending strength and bearing strength) of the rack is improved, and a rack and pinion mechanism with strength sufficient to withstand the torque load from motor inertia can be achieved. The part of the rack shaft where the rack is not formed only needs rigidity comparable to a conventional rack shaft because it simply slides to push the wheels for steering. The weight of the rack shaft can also be limited because it is only necessary to increase the tooth width of the rack.
It is further preferable to make the tooth width of the rack formed on the rack shaft greater than the diameter of the rack shaft in that part where the rack is not formed.